The present invention relates to mechanical circulatory support devices or “MCSDs.” MCSDs are used to assist the pumping action of the heart. Certain MCSDs are used to assist the pumping action of a ventricle of the heart and, therefore, are referred to as “ventricular assist devices” or “VADs.” For example, as shown in U.S. Pat. Nos. 7,972,122; 8,007,254; and 8,419,609, the disclosures of which are hereby incorporated by reference herein, one form of MCSD incorporates a generally cylindrical inner casing defining a flow path and a rotor mounted within the flow path for rotation about the axis of the flow path. The rotor is arranged to impel blood along the axis of the flow path. Electrical coils are mounted around the inner casing, and an outer casing surrounds the electrical coils. The coils provide a rotating magnetic field within the flow path. The rotor has a permanent magnetization that interacts with the rotating field so that the rotating field impels the rotor in rotation about the axis. The MCSD may include a volute that serves to redirect the flow from the axial direction to a direction transverse to the axis. The volute has an outlet connection that serves as the outlet connection of the MCSD.
Such a pump can be implanted within the thoracic cavity of a human patient as, for example, within the pericardial sack. The inlet end of the housing may be connected directly to the ventricle or connected to the ventricle by a short inlet cannula. The outlet connection may be connected, for example, to the aorta by an outlet cannula. Merely by way of example, a typical MCSD of this type has a capacity to pump about 7-10 liters per minute against a pressure difference or head of about 75 mm Hg, and thus can bear a substantial proportion or almost all of the pumping load typically carried by the left ventricle. Merely by way of example, the outer casing of such a pump may be about 21 mm in diameter, and the volute may have somewhat larger dimensions in a plane perpendicular to the axis.
Such intra-thoracic implantation requires invasive surgery, commonly requiring open-chest surgery which likely includes a sternotomy. Further, such positioning requires attaching the MCSD directly to the heart or within a chamber of the heart. While such procedures are high risk for any patient, they are particularly dangerous for patients in need of such MCSDs as they are typically in poor health and less likely to successfully undergo such surgeries.
Other MCSDs, such as those shown in U.S. Pat. Nos. 7,905,823 and 8,768,487 and in U.S. Patent Application Publication No. 2014/0275723, the disclosures of which are hereby also incorporated by reference herein, include generally similar elements but are of smaller size. These MCSDs typically implanted outside of the thoracic cavity as, for example, under the skin within the soft tissues of the pectoral area. These devices typically are connected to the heart as, for example, to the left atrium by an inlet cannula extending from the location of the pump to the atrium. The outlet of the pump typically is connected to an artery as, for example, the subclavian artery. Because the pump is implanted outside of the thoracic cavity, remote from the heart, the implantation procedure is considerably less invasive. Typically, the cannula can be inserted into a chamber of the heart by a laparoscopic or catheter-based procedure and threaded through the tissues of the body to the location of the pump. The procedure for inserting the outlet cannula is also performed outside of the thoracic cavity. Moreover, because the pump is located outside of the thoracic cavity, the pump can be accessed readily if it becomes necessary to repair or replace it.
MCSDs intended for extra-thoracic placement typically have been configured to provide lower pumping capacity than MCSDs intended for intra-thoracic implantation. For example, a typical MCSD intended for extra-thoracic implantation may provide a blood flow of about 1-4 liters per minute at a 75 mm Hg head. These MCSDs thus carry a smaller proportion of the pumping load of the heart. Such pumps typically have smaller dimensions than pumps intended for intra-thoracic implantation.
Extra-thoracically implanted MCSDs typically are housed in a pocket within the soft tissues outside of the thoracic cavity. Such pockets normally are created by surgical procedures as, for example, separating skin or subcutaneous fat from the underlying muscular tissue or separating layers of muscular tissue from one another. In some instances, the tissues forming the wall of a pocket surrounding an extra-thoracic MCSD can erode. Such erosion arises from mechanical action of the MCSD against the surrounding tissues. Mechanical action of the MCSD can lead to inflammation and necrosis of the tissues surrounding the pocket, and can cause the pocket to become enlarged. This difficulty can be particularly pronounced where the pocket closely overlies bones such as ribs. Enlargement of the pocket may allow movement of the MCSD, which creates an uncomfortable sensation for the patient. In severe cases, these conditions may require correction by additional surgical procedures.
Certain aspects of the present invention provide MCSDs and implantation methods that can address these concerns. Moreover, the improved MCSDs and implantation methods may allow implantation of larger MCSDs in extra-thoracic locations. For example, MCSDs of the type typically used heretofore for intra-thoracic implantation can be implanted extra-thoracically.
Currently available MCSDs that can accommodate intra-thoracic MCSD pumping capacity present drawbacks when used for extra-thoracic positioning due at least to the size and shape of the MCSD. For instance, the width of the volute of the aforementioned intrathoracic pump can pose difficulties in extra-thoracic implantation. As such, there is a need in the art for alternative MCSD designs that are suitable for intra-thoracic volumetric pumping capacity while being capable of extra-thoracic positioning within a patient.